Swash plate devices

ABSTRACT

This invention concerns a swash plate device having a rotary cylinder block, cylinders in the block arranged parallel to or inclined to the block rotation axis, pistons reciprocable in the cylinders and projecting from one end of the cylinder block, a tiltable swash plate disposed adjacent the one end of the block and having a flat swash surface engaged by the pistons, and a valve surface having a high pressure port and a low pressure port connecting to and disconnecting from cylinder ports during block rotation, the arrangement being such that during cylinder block rotation each piston is urged partly into its cylinder to precompress liquid therein after the cylinder disconnects from the low pressure port before it connects to the high pressure port. Such precompression reduces noise in operation of the swash plate device but causes an alternating tilting moment to be applied to the swash plate depending on the operating pressure of the device. The present invention provides that the tilt axis of the swash plate should lie in between the line of action of the thrust of each piston on the flat swash surface at the extreme end of its stroke in its cylinder as it leaves the low pressure port and a line which passes perpendicularly through the articulation plane of the pistons at the point of intersection with the cylinder block axis.

United States Patent McLeod Nov. 27, 1973 SWASH PLATE DEVICES Donald Patterson McLeod, Cheltenham, England [75] Inventor:

[73] Assignee: Dowty Technical Developments Limited, Cheltenham,England [22] Filed: Mar. 1, 1971 [21] Appl. No.: 119,605

Related US. Application Data [63] Continuation-in-part of Ser. No. 33,716, May 1,

Primary Examiner-William L. Freeh I Assistant ExaminerGregory LaPointe Attorney-Young & Thompson [5 7] ABSTRACT This invention concerns a swash plate device having a rotary cylinder block, cylinders in the block arranged parallel to or inclined to the block rotation axis, pistons reciprocable in the cylinders and projecting from one end of the cylinder block, a tiltable swash plate disposed adjacent the one end of the block and having a flat swash surface engaged by the pistons, and a valve surface having a high pressure port and a low pressure port connecting to and disconnecting from cylinder ports during block rotation, the arrangement being such that during cylinder block rotation each piston is urged partly into its cylinder to precompress liquid therein after the cylinder disconnects from the low pressure port before it connects to the high pressure port. Such precompression reduces noise in operation of the swash plate device but causes an altemating tilting moment to be applied to the swash plate depending on the operating pressure of the device. The

present invention provides that the tilt axis of the swash plate should lie in between the line of action of the thrust of each piston on the flat swash surface at the extreme end of its stroke in its cylinder as it leaves the low pressure port and a line which passes perpendicularly through the articulation plane of the pistons at the point of intersection with the cylinder block axis.

8 Claims, 11 Drawing Figures SWASI-I PLATE DEVICES This is a continuation in part from application Ser. No. 33,716.

This invention relates to a swash plate device comprising a rotary cylinder block having cylinders therein either parallel to or slightly inclined to the rotation axis, a tiltable swash plate, pistons in the cylinders cooperating with the swash plate whereby the pistons will reciprocate during cylinder block rotation, and valve means co-operating with the block to connect the cylinders during block rotation alternately to a fixed high pressure port and to a fixed low pressure port in the valve means. i

In this kind of swash plate device it is conventional for the tilt axis of the swash plate to be located so that it lies in the plane of articulation of the pistons relative to the swash plate and also intersects the cylinder block axis. For the purpose of this specification the plane of articulation of the pistons is defined as that plane containing the centres of articulation movement of each piston relative to the fiat swash plate surface. Where there are ball jointed slippers interposed between the pistons and the swash plate surface the plane of articulation passes through the centres of the ball joints.

The main high and low pressure ports inthe valve means are conventionally arranged to connect to the cylinders so that one main port connects to each cylinder while its piston moves from its outermost to its innermost position and the other main port connects to each cylinder while its piston moves from its innermost to its outermost position. It is known with this conventional valve arrangement that when the high pressure port makes connection with each cylinder a pulse of noise is developed as each cylinder becomes rapidly charged with high pressure liquid. It is also known that this effect may be reduced if the high pressure port is so phased relative to piston reciprication that each piston is actually moved slightly into its cylinder after disconnection from the low pressure port before connec-. tion is made to the high pressure port. Liquid is thus precompressed in each cylinder but with the disadvantage of applying alternating tilting moments to the swash plate making it necessary for the servo or other means controlling the swash plate to be double-acting. Tilting moment alternates between predominantly piston stroke increasing at low operating pressures and predominantly piston stroke decreasing at high operating pressures.

The present invention sets out to provide a simple swash plate device whose valve arrangement provides for precompression to give noise reductionand which provides that a uni-directional moment or an almost uni-directional moment will be generated on the swash plate by the pistons thus enabling a simple swash angle control to be used capable of exerting force in one direction only.

In accordance with the present invention a swash plate device comprises a rotary cylinder block, cylinders inthe block arranged parallel to or inclined to the block rotation axis, pistons reciprocable in the cylinders and projecting from one end of the cylinder block, a tiltable swash plate disposed adjacent the one end of the block and having a flat swash surface engaged by the pistons, and a valve surface having a high pressure port and a low pressure port conn ecting to and disconnecting from cylinder ports during block rotation, the

tilt axis of the swash plate, the flat swash surface, the cylinder block axis and the high pressure port being relatively positioned during operation at the maximum swash angle so that each piston during cylinder block rotation is urged partly into its cylinder after the cylinder disconnects from the low pressure port and before it connects to the high pressure port, the tilt axis of the swash plate lying in between the line of action of the thrust of each piston on the flat swash surface at the extreme end of its stroke as its cylinder disconnects from the low pressure port and a line which passes perpendicularly through the articulation plane of the pistons at the point of intersection with the cylinder block axis.

For preference the tilt axis of the swash plate should pass through a triangle formed by the line of action of the thrust of each piston onto the flat swash surface at the extreme end of its stroke as its cylinder disconnects from the low pressure port, the rotation axis of the cylinder block and a line in the articulation plane between the said rotation axis and the said line of action.

A single-acting hydraulic servo motor may be connected for adjustment of the swash plate angle and may employ liquid from the said high pressure port.

The swash plate tilt axis may lie at right-angles to a radial plane through the cylinder block axis and the fiat swash surface may be parallel to the tilt axis.

Preferably the swash plate tilt axis is offset from the flat swash surface on the side thereof remote from the pistons so that a single trunnion shaft may extend through the swash'plate without intersecting the swash surface to determine the swash plate tilt axis.

, The pistons may engage the fiat swash surface through the medium of slippers universally jointed one to each piston.

The cylinder block may include only five cylinders and pistons. Preferably the urging of each piston into its cylinder after disconnection from the low pressure port and before connection to the high pressure port generates a total precompression of about one half of the maximum working pressure at the high pressure port.

Preferably the line of action of the thrust of each piston on the swash plate at maximum swash angle at the extreme end of its stroke as its cylinder disconnects from the low pressure port passes the tilt axis at a distance less than half the diameter of the piston.

One embodiment of the invention for use as a pump will now be particularlydescribed with reference to the accompanying drawings, in which,

FIG. 1 is a cross-section through the pump,

FIG. 2 is a composite elevation and cross-section showing in part an elevation of the pump and part sections on the lines IIA, [IE and IIC of FIG. 1,

FIG. '3 is an elevation of the end surface of the cylinder block containing the cylinder ports,

FIG. 4 is an elevation of the structural member showing in particular the flat valve surface,

FIG. 5 is a developed cross-section taken on the circular line VV in FIG. 4,

FIGS. 6 and 7 are porting diagramsshowing different port positions,

FIGS. 8 and 9 are diagrams showing piston positions corresponding to FIGS. 6 and 7 and FIGS. 10 and 11 are graphs plotting swash plate moment against cylinder block angle.

In the drawings the structural member of the pump is an aluminium casting and it comprises three integrally formed parts indicated as the valve 1, the casing 2 and the servo cylinder 3. The valve 1 has a flat valve surface 4 which contains a high pressure delivery port 5 of conventional kidney shape connected to a delivery passage 6 which terminates as a screw-threaded connection (not shown) on the outside of the valve 1. A plurality of mounting lugs 8 are provided around the periphery of the valve 1 for securing the pump in its operative position, the pump being correctly located by a cylindrical boss 9 adapted to fit in a corresponding hole on a driving motor or engine. The hollow casing 2 is closed at the end remote from the valve 1 by means of a cover 1 l of pressed sheet metal secured in a large circular recess 12 by means of a circlip 13. A rubber seal 14 ensures a liquid tight connection between the casing 2 and the cover 11.

In the casing 2 a cylinder block 15 of cast iron is arranged for rotation about the axis A-A. The cylinder block 15 includes a flat surface 16 bearing against the valve surface 4. Within the block there are five regularly spaced cylinders 17 all of whose axes are parallel to the rotation axis AA of the block. Each cylinder 17 includes a cylinder port 18 opening into the surface 16, the ports 18 being arranged to co-operate with the main pressure port 5 during rotation of the cylinder block 15. At a position remote from the surface 16, the cylinder block includes an integrally formed skirt 19 around which is formed a cylindrical bearing surface 21 which effectively surrounds the cylinder block. Within the casing 2 adjacent to the cylindrical bearing surface 21 an internal cylindrical bearing surface 22 is formed which directly engages the cylindrical bearing surface 21 of the block to locate the block for rotation.

Five passages 24 are formed within the cylinder block one between each adjacent pair of cylinders. Each passage opens from the surface of the block remote from the valve surface 4 between the radially directed vanes 25 formed on the cylinder block 15 within the casing 2. A groove 26 is formed in the valve around the valve surface 4, one end of this groove connecting to the low pressure arcuate inlet port formed in the valve surface 4. I

The casing 2 remote from the valve includes a pair of integrally formed bosses 27 bored transversely to produce a pair of spaced apertures 28. Between the bosses 27 a swash plate 29 is mounted on a shaft 31 by means of a cylindrical bore 32 extending through the swash plate. The axis of shaft 31 lies at right angles to a plane through the cylinder block axis. The swash plate includes a flat swash surface 33 facing the cylinder block 15 and parallel to the axis of shaft 31. Within each cylinder 17 a piston 34 and a compression spring 35 are located, the spring acting to urge its piston outwardly from the cylinder. At its outer end each piston 34 is formed with a spherical socket 36 into which a ball 37 is secured. The ball 37 is integrally connected to a slipper 38 engaging the swash surface 33. A hydraulic passage 39 within each piston 34 gives access to the socket 36 for lubrication and also connects to passage 41 in the ball which feeds at pressure through a restrictor 42 to a recess 43 in the slipper surface in contact with the swash surface 33. The recess is conventionally arranged so that hydraulic pressure acting in the recess will almost completely balance the hydraulic load. acting on the piston.

A retaining plate 44 includes five apertures 45 engaged one on each slipper 38. Centrally the retaining plate 44 reacts against a ball 46 carried by a pad 47 slidably mounted in a central bore in the cylinder block, a compression spring 49 reacting between the block 15 and the pad 47 and serving simultaneously to urge the block against the valve surface 4 and to urge the retaining plate 44 to maintain the slippers in contact with the swash surface.

A splined aperture 51 is provided centrally within the cylinder block to open into a comparatively large aperture 52 extending through the valve 1. A drive shaft (not shown) may be inserted through the aperture 52 to engage the splined aperture to drive the cylinder block. A seal 53 may engage between the valve and the cylinder block in the manner disclosed in U.S. Pat. No. 3,636,820. To prevent hydraulic leakage between the bore 48 and the aperture 52 the bore 48 is sealingly closed by means of a plug 54.

The servo cylinder 3 has a servo piston 55 slidably mounted therein one end being connected by means of a pivot pin 56 to a link 57 which in turn is connected by a pivot pin 58 to a lever 59 extending from the swash plate 29. In FIG. 1 the swash plate is shown at a position of maximum inclination and the servo piston 55 is in a corresponding position.

The small diameter bore 61 within the piston 55 receives one end of a rod 62 which extends from the end of the servo cylinder 3 adjacent to the valve 1. The end of the cylinder 3 adjacent to the valve 1 has a plug 63 inserted therein being retained in position by a cap 64 and a circlip 65. A central bore 66 through the plug 63 locates the rod 62 for sliding movement, a suitable seal on the rod 62 preventing leakage of liquid. The end of the cylinder 3 adjacent to the valve 1 is formed in an extension 67 from the valve 1 but integral therewith. A rotary controlled spindle 68 is suitably mounted in bearings in the extension 67, the spindle 68 carrying a lever 69 which extends into a chamber 71 within the plug 63 to engage in a recess 72 in the end of the rod 62 such that rotation of the spindle 68 will cause endwise movement of the rod 62. A passage 73 extends in the valve 1 from the delivery passage 6 to intersect the cylinder bore 3. At the position of intersection the plug 63 includes a peripheral groove 74 from which a small hole is bored to form the restrictor 75 which connects through passage 76 in plug 63 to the working space 77 between the plug 63 and the piston 55.

The end of the rod 62 includes a counter bore 78 from which a pair of radially directed control ports 79 open into the working space 77. The rod 62 is located for limited motion within the piston bore 61 by means of a transverse pin 81 loosely engaging a transverse hole in the rod 62. The movement permitted for the rod 62 relative to the piston will vary the opening of the port 79 over the edge of the bore 61 into the working space 77. The ports 79 and the co-operating edge of the bore 61 form a variable restrictor adjustable by movement of rod 62.

The end of the servo bore 3 opposite to the valve 1 is formed as a screw-threaded inlet connection 82 and liquid entering connection 82 may pass into the casing through a cut-away portion 83 through which the lever 59 extends to the link 57. The liquid entering the pump thereby flows firstly over the swash plate 29 before entering the passages 24.

FIG. 3 of the accompanying drawings shows the flat end surface 16 of the cylinder block in elevation. The splined central hole 51 is surrounded by the flat block surface 16 which contains the cylinder ports 18, the flat surface 16 being interrupted by a co-axial circular groove 88 radially located in between the aperture 51 and the cyliner ports 18. The surface 16 is surrounded by an annular flat surface 89 parallel to but slightly lower than the flat surface 16. The annular surface 89 includes a plurality of shallow recesses 91 which define between them the axially extending vanes 92 for propelling liquid.

The annular surface 89 fits over the groove 26 (FIG. 4) formed around the flat valve surface 4. The groove 26 is closed by a wall 93 which extends from the valve surface 4 across the groove 26 and connects to a land 94 to reduce the width of the groove 26 at the end portion thereof adjacent the wall. The groove 26 at its narrowed portion connects to one end of the low pressure inlet port 30. As seen in FIG. 5 the groove 26 starts at a shallow depth 93 and gradually increases in depth around the valve surface 4, maximum depth of the groove being obtained at the position of entry to the inlet port 30. The narrowed part of groove 26 adjacent to the land 94 increases more greatly in depth than the remainder of the groove 26. The groove 26 and its operation are described in more detail in US. Pat. No. 3,669,568.

The seal 53 is seated in an annular recess 95 formed in the centre of the valve 1 so that the seal 53 can engage the flat portion of the blocksurface 116 between the central aperture 51 and groove 88. The outer edge of the recess 95 is chamfered at 96 so that a portion of the recess 95 radially outwards of the seal 53 makes connection with the annular groove 88 in the cylinder block surface 16. A shallow channel 97 in the valve surface 4 connects from the recess 95 into the inlet port 30 whereby liquid at low pressure built up in the inlet port 30 may enter the recess 95 around the sea] 53 to retain a low backing pressure at the seal 53 to help to prevent induction of air from the aperture 52 across the seal and into the pump. The groove 88 and its operation are described in more detail in US. Pat. No.

In operation the cylinder block is rotatablydriven by a drive shaft inserted through the aperture 52 into the cylinder block and rotation of the cylinder block causes reciprocation of the pistons in the cylinders by virtue of the fact that the slippers are held by springs 35 and 49 against the swash plate surface. During rotation liquid fed from the inlet connection 82 flows around the swash plate and into the passages 24 there being induced by centrifugal pumping action to flow to the groove 26. The rotation of the cylinder block surface 89 over the groove 26 will by virtue of the vanes 92 induce flow of liquid around the groove 26 and into the inlet port 30.

The total pressure build up at the inlet port 30 results from the centrifuging action of the passages 24 and vanes 25 of the cylinder block and further from the impelling action of the vanes 92 on liquid in groove 26 by virtue of the viscosity of the liquid. The pressure of liquid in the inlet port 30 whilst still comparatively low will nevertheless provide an adequate boost pressure to assist the entry of liquid into the cylinders 17 during outward movement of the pistons 34. It will be understood that for the direction of rotation of the cylinder block, pistons in cylinders connected to the inlet port 30 will be moving outwardly from their cylinders whilst pistons in cylinders connected to the delivery port 5 will be moving into their cylinders to compress the liquid and deliver it to the port 5.

The offset relation of the axis B-B of shaft 31 having regard to the rotation axis AA of the cylinder block results in a torque being generated on the swash plate 29 about the axis of shaft 31 and in proportion to the high pressure delivered at port 5 which will tend to reduce the inclination of the swash plate to zero, the limit of such movement being the engagement of head 84 of rod 62 against the cap 64. Maximum inclination of the swash plate is limited by the engagement of the head 84 against the end of working space 71 opposite to the cap 64. The required angular setting for the swash plate is selected by the spindle 68 which adjusts the axial position of the rod 62. Movement of the rod 62 will adjust the variable restrictor formed by port 79 to control the pressure developed within the working space 77 by virtue of the series connection of the fixed restrictor 75 with the variable restrictor between the high pressure connection 76 and the low pressure interiorly of the casing 2. The pressure within the working space 77 which is dependent on high pressure from port 5 urges the swash plate towards its maximum inclination whilst the torque developed on the swash plate itself urges it to the minimum inclination. Simple adjustment of the spindle 68 will therefore control pressure in the working space 77 to cause the swash plate to take up an angular position which is in proportion to the angular setting of the spindle 68. It will be seen that the piston 55 and cylinder 3 together form a single-acting servomotor having only one working space 71.

It will be appreciated particularly from FIG. 4 that the high pressure port 5 is so positioned that it makes connection with each cylinder port 18 only after the associated piston has passed its bottom dead centre position and is being urged by the swash plate into the cylinder. FIGS. 6 and 7 illustrate more clearly the relative positions of the high pressure port 5 and the cylinder ports 18. For convenience in identification the cylinder ports 18 are indicated as slightly wider in the radial sense than the high and low pressure ports 5 and 30. It will, however, be appreciated that in practice it is preferred for these ports to be of exactly the same width. FIG. 6 shows the relative positions in which a cylinder port 18 having just passed the bottom dead centre position is just dis-connecting from the low pressure port 30. FIG. 7 illustrates that the cylinder block has moved through a small angle to bring the same cylinder port 18 to a position where it is just about to make connection with the high pressure port 5. This angle is referred to as the precompression angle and for movement over this angle the swash plate will have urged the piston slightly into its cylinder whilst the cylinder is completely closed by the valve surface, thus compressing the liquid and raising its pressure. The precompression angle is carefully selected by experiment having regard to the working pressure at the high pressure port 5, and the maximum angle of theswash plate. In the illustrated example the swash plate is intended for operation at 3,000 psi. and at a maximum swash angle of 17. For these operating conditions the precompression angle has been selected at about 16 and this is sufficient to raise the pressure within each cylinder to about half the maximum working pressure when at full swash angle. It will be appreciated that with reduction of swash angle the degree of precompression will reduce and will be roughly in proportion to the swash angle.

FIGS. and 11 are graphs setting out the moments exerted on the swash plate. FIG. 10 represents the moments exerted on the swash plate by one piston both for the pump as illustrated and for a conventional swash plate pump of the dimensions of the illustrated pump but having its pivot axis for the swash plate in its conventional position, i.e. passing through the point of intersection of the rotation axis in the articulation plane. The swash plate moment is indicated in pounds inches. The full lines in FIG. 10 represent swash plate moments for a conventional pump and the dotted lines represent the swash plate moments for the pump shown in the drawings. The full line indicated at 01 represents the swash plate moments within which a piston connects to and disconnects from the high pressure port working at 300 p.s.i. The initial part of curve 01 shows a rapid rise in pressure representing cylinder movement through the precompression angle and a rapid drop in pressure as the cylinder connects to the high pressure port. Curve 02 represents the swash plate moment for the conventional pump when the operating pressure is at 3,000 p.s.i. It will be noted that there is the same pressure rise at the precompression angles as for curve 01 but after this point there is a further pressure rise to a point representing the maximum pressure. The precompression is so chosen that about 1,500 p.s.i. is achieved during precompression. Curve N1 represents the moments for the pump shown in the drawings when the operating pressure is 300 p.s.i. the great reduction in the moment on the swash plate through the precompression angle will be noted but it will be appreciated that the actual precompression pressure is the same for all curves in FIG. 10. Curve N2 shows the swash plate moment generated for the pump in the drawings when the operating pressure is at 3,000 p.s.i.

FIG. 11 shows the total moments over 360 of cylinder block rotation applied to the swash plate both for the conventional pump having the pivot axis in the conventional position and for the pump of the drawings. Again the full lines represent the conventional pump and the dotted lines represent the pump of the drawings. Curve 03 represents the moments for the conventional pump when the operating pressure is at 300 p.s.i. The peaks will be noted which correspond to the precompression angles for each of the five cylinders. In particular it will be noted that these peaks are positive, i.e. the moment tends to increase the swash angle. The corresponding curve for the pump of the drawings is shown as N3 and it will be appreciated here that whilst the peaks corresponding to precompression are still present they are very much reduced from that of the conventional pump and in particular the curve remains negative continuously i.e. the swash plate is continuously urged in the displacement reducing direction. Curve 04 represents the moments generated by the conventional pump when operating at 3,000 p.s.i. It

a. The swash plate moment of the conventional pump alternates at low delivery pressure although it remains predominantly positive.

b. The swash plate moment of the conventional pump for high delivery pressure is mainly negative. Therefore to control the conventional pump the servo motor must be capable of exerting a controlling force for both directions i.e. it must be double acting.

c. In the swash plate pump of the drawings the swash plate moments at both low and high operating pressures are negative, the average moment being in each case roughly proportional to the operating pressure. Moments due to precompression are reduced or eliminated. A simple servo motor having only one working space and fed with liquid at the pressure in the high pressure port may therefore operate to control the pump of the drawings. The actual size of such a servo motor is not necessarily larger than a servo motor necessary to control the swash plate of the conventional pump since the moments due to precompression generated in the conventional pump necessarily must be controlled even at low operating pressures and if the servo motor is operated by the pressure available at the high pressure port then the servo motor must be of substantial size.

Attention is now directed particulary to FIGS. 8 and 9. In FIG. 8 the lower piston 17 has its associated cylinder port as in FIG. 6 at the lower position and therefore this piston is just begining its precompression movement. FIG. 9 illustrates the piston position corresponding to the cylinder port arrangement of FIG. 7, where the cylinder block has moved through 16 from the FIG. 6 position to effect precompression. The lower piston 17 is just about to connect to the high pressure port and therefore has its maximum precompression. The direction of piston thrust on the swash plate is shown by the line EE in FIG. 8 and the line F-F in FIG. 9 and in each case extends from the centre of the ball 37 at right angles to the swash plate surface 33. As shown the swash plate angle is at the maximum and it will be seen that the distance d between the line EE and the tilt axis of the swash plate is less than half the diameter of a piston. The smaller the distance d the smaller will be the moment which precompression pressures can exert on the swash plate. The spacing of the line F-F from the tilt axis is only very slightly less than the distance d in the illustrated embodiment.

The axis of trunnion 31 is offset in two senses to attain its illustrated position. Firstly, it is offset from the plane of articulation of the pistons i.e. the plane containing the centres of the ball joints 37, this offset being illustrated at the dimension e in FIG. 8. Secondly, the trunnion axis is offset from the rotation axis A'A as illustrated at dimension f in FIG. 8. It will be appreciated that within the invention there is a range of variation possible for the two offsets e and f. Each of the two offsets e or f may be reduced to zero provided that the other offset is present in a substantial degree.

The main criterion, however, is that the tilt axis should lie in between two lines determined at the maximum swash angle of the swash plate and here reference is made parrticularly to FIG. 8. The first line, indicated at DD extends from the inter-section of the rotation axis A--A with the articulation plane in a direction perpendicular to the articulation plane. The second line indicated at EE extends from the centre of a ball joint whose piston is at the extreme end of its stroke and is just starting precompression in its cylinder, in a direction perpendicular to the articulation plane and inter-secting axis AA. For any position of the tilt axis between lines D-D and E-E the precompression force exerted on the swash plate successively by each piston produces either a negligible moment about the tilt axis or a moment which is opposed to the moments produced by one or more pistons in connection with the high pressure port 5, to ensure that the total moment exerted on the swash plate is predominantly unidirectional over a substantial range of operating pressures and is also moderate in magnitude enabling the servo for controlling the swash angle to be single-acting and of moderate size.

For economy in manufacture it is desirable to be able to use only one shaft 31 to support the swash plate for tilting movement. Such a shaft must then be located so as not to intersect the swash surface and the offset 2 is then made substantial in size whilst maintaining the tilt axis between the lines DD and EE.

For ensuring uni-directional moment on the swash plate at very low operating pressures and at maximum swash angle it is desirable to arrange that the tilt axis passes within the triangle formed by the line EE, the rotation axis A-A, and a line GG in the articulation plane between axis A--A and a ball joint centre at the bottom dead centre position. The swash plate moment due to precompression is then less than the opposing swash plate moment due to the low operating pressure and thereby enables the swash plate device to operate at quite low pressures without reversal of swash plate moment. If it is desired to use a single shaft 31 to mount the swash-plate then the tilt axis must pass through a triangle forrned by lines E-E, rotation axis AA and the line in the swash plate surface 33 joining the intersec tion with line E-E and axis AA.

The invention is of particular importance in a fivecylinder swash plate device since the swash plate moment can be made substantially uni-directional over the range of operating pressures whereas a conventional structure of a five-cylinder swash plate pump would produce severe alternating swash plate moments.

In the described embodiment the valve surface. has been shown as a fixed valve surface incapable of any adjustment and accordingly when the illustrated device is operating the advantages previously described will occur onlywhen the cylinder block rotates in the direction to produce the described precompression. It is, however, within the scope of the present invention to arrange a swash plate device which is rotatable in either direction, provision being made for adjustment of the high and low pressure valve ports in the valve surface appropriate to the direction of rotation to give the required precompression.

Also in the described embodiment the tilt axis of th swash plate has been shown as lying perpendicular to a radial plane through the rotation axis and the swash surface of the swash plate has been shown as parallel to the tilt axis. It is, however, within the scope of the present invention for the tilt axis to be slightly inclined to the perpendicular radial plane through the rotation axis and/or for the swash surface to be slightly inclined to the tilt axis. These slight inclinations have been previously proposed in swash plate'pumps with the intention of'varying the degree of precompression with variation in swash angle.

Whilst the described embodiment is intended for use as a pump the present invention may also be applied to a swash plate motor.

It is also within the scope of the present invention to provide a restricted flow passage in the valve surface extending from the high pressure port towards the low pressure port such that as each cylinder port passes from the low pressure port towards the high pressure port during precompression, it makes a connection to the restricted flow passage permitting restricted liquid flow into, or out of the cylinder in dependence on the pressure in the high pressure port. Such a restricted flow passage is indicated at in FIGS. 4, 6 and 7. Precompression is thus modified in accordance with the working pressure at the high pressure port, but the location of the swash plate tilt axis in accordance with the invention as claimed will operate to provide for the swash plate either a uni-directional tilt moment or a moment more nearly uni-directional than would be generated if the tilt axis were located in its conventional position.

I claim:

1. A swash plate device comprising a rotary cylinder block, cylinders in the block extending in the direction of the block rotation axis, pistons reciprocable in the cylinders and projecting from one end of the cylinder block, a tiltable swash plate disposed adjacent the one end of the block and having a flat swash surface engaged by the pistons, and a valve surface having a high pressure port and a low pressure port connecting to and disconnecting from cylinder ports during block rotation, the tilt axis of the swash plate, the flat swash surface, the cylinder block axis and the high pressure port being relatively positioned during operation at the maximum swash angle so that each piston during cylinder block rotation is urged partly into its cylinder after the cylinder disconnects from the low pressure port and before it connects to the high pressure port, the tilt axis of the swash plate passing through a triangle formed by the line of action of the thrust of each piston onto the flat swashsurface at the extreme end of its stroke in its cylinder as the cylinder disconnects from the low pressure port, the rotation axis of the cylinder block and a line in the articulation plane between the said rotation axis and the said line of action, said tilt axis being spaced from said rotation axis in the direction of said line of action.

2. A swash plate device as claimed in claim 1, wherein the tilt axis of the swash plate passes through and is spaced from the sides of a triangle formed by the line of action of the thrust of each piston onto the flat swash surface at the extreme end of its stroke as the cylinder disconnects from the low pressure port, the rotation axis, and a line in the swash plate surface between the rotation axis and the said line of action.

3. A swash plate device as claimed in claim ll, including a single-acting hydraulic servo motor connected for adjustment of the swash plate angle, the hydraulic liquid for operating the said servo being the liquid available at said high pressure port.

4. A swash plate device as claimed in claim ll, wherein the swash plate tilt axis is offset from the flat swash surface on the side thereof remote from the pistons and including a single shaft extending through the swash plate without intersecting the swash surface to determine the swash plate tilt axis.

5. A swash plate device as claimed in claim 1, wherein the swash plate tilt axis is offset from the flat swash surface on the side threof remote from the pistons and including a single shaft extending through the swash plate without intersecting the swash surface to determine the swash plate axis.

6. A swash plate device as claimed in claim I, having five cylinders only in the cylinder block containing pistons which engage the swash plate surface.

7. A swash plate device as claimed in claim 1, including a restricted flow passage in the valve surface extending from the high pressure port towards the low pressure port such that as each cylinder port passes from the low pressure port to the high pressure port it makes connection to the restricted flow passage before making connection with the high pressure port.

8. A swash plate device as claimed in claim 1, wherein the pistons engage a flat swash surface through the medium of slippers, there being one slipper universally jointed to each piston, the centre of universal jointing of each piston lying on the said articulation plane, and the tilt axis of the swash plate lying on the side of the flat swash surface, remote from said articulation plane. 

1. A swash plate device comprising a rotary cylinder block, cylinders in the block extending in the direction of the block rotation axis, pistons reciprocable in the cylinders and projecting from one end of the cylinder block, a tiltable swash plate disposed adjacent the one end of the block and having a flat swash surface engaged by the pistons, and a valve surface having a high pressure port and a low pressure port connecting to and disconnecting from cylinder ports during block rotation, the tilt axis of the swash plate, the flat swash surface, the cylinder block axis and the high pressure port being relatively positioned during operation at the maximum swash angle so that each piston during cylinder block rotation is urged partly into its cylinder after the cylinder disconnects from the low pressure port and before it connects to the high pressure port, the tilt axis of the swash plate passing through a triangle formed by the line of action of the thrust of each piston onto the flat swash surface at the extreme end of its stroke in its cylinder as the cylinder disconnects from the low pressure port, the rotation axis of the cylinder block and a Line in the articulation plane between the said rotation axis and the said line of action, said tilt axis being spaced from said rotation axis in the direction of said line of action.
 2. A swash plate device as claimed in claim 1, wherein the tilt axis of the swash plate passes through and is spaced from the sides of a triangle formed by the line of action of the thrust of each piston onto the flat swash surface at the extreme end of its stroke as the cylinder disconnects from the low pressure port, the rotation axis, and a line in the swash plate surface between the rotation axis and the said line of action.
 3. A swash plate device as claimed in claim 1, including a single-acting hydraulic servo motor connected for adjustment of the swash plate angle, the hydraulic liquid for operating the said servo being the liquid available at said high pressure port.
 4. A swash plate device as claimed in claim 1, wherein the swash plate tilt axis is offset from the flat swash surface on the side thereof remote from the pistons and including a single shaft extending through the swash plate without intersecting the swash surface to determine the swash plate tilt axis.
 5. A swash plate device as claimed in claim 1, wherein the swash plate tilt axis is offset from the flat swash surface on the side threof remote from the pistons and including a single shaft extending through the swash plate without intersecting the swash surface to determine the swash plate axis.
 6. A swash plate device as claimed in claim 1, having five cylinders only in the cylinder block containing pistons which engage the swash plate surface.
 7. A swash plate device as claimed in claim 1, including a restricted flow passage in the valve surface extending from the high pressure port towards the low pressure port such that as each cylinder port passes from the low pressure port to the high pressure port it makes connection to the restricted flow passage before making connection with the high pressure port.
 8. A swash plate device as claimed in claim 1, wherein the pistons engage a flat swash surface through the medium of slippers, there being one slipper universally jointed to each piston, the centre of universal jointing of each piston lying on the said articulation plane, and the tilt axis of the swash plate lying on the side of the flat swash surface, remote from said articulation plane. 